ah ok, it's just very different to how all other tags are used (no bech32 encoding) so supporting that would require changing libraries and querying for two types of indexed values (i.e. the way NIP-01 describes it and this new encoding) 

 Okay, I was trying to avoid adding another tag, but I guess I should avoid single-letter tags. I could call it "navigation", or something. 

 it's not the a tag, it's the encoding of the tag that is very non-standard (i.e. doesn't follow NIP-01); I'm guessing some relays might bark at the non-standard format 

 Yeah, I need the eventID in there, unfortunately. Can't go with pubkey.